The winner of the primate hide and seek competition (if you don't count Bigfoot) may be the bald-faced Vanzolini saki. This large, long-tailed monkey was collected and named in 1936 in Columbia, then vanished in a puff of leaves and was never seen again.
An expedition led by Dr. Laura Marsh spotted the animal swinging in the trees beside the Eiru River. The habitat is in pretty good shape, but like virtually all primate habitat, requires intelligent protection from illegal logging, hunting, and deforestation to preserve the saki and its co-inhabitants.
Nice to get a win for conservation!